1. Field of the Invention
The present invention relates generally to video signal enhancement apparatus and more particularly, to noise clippers for reducing the noise contamination of a video enhancement signal.
2. Description of the Prior Art
Because of bandwidth limitations in video equipment, signals representing abrupt transitions in the luminosity of images are not faithfully reproduced. Images created from these bandwidth limited signal manifest a resultant lack of detail. To compensate for such loss of detail, video enhancement circuits are used which combine the video signal with the inverse of the second derivative of that signal. The resultant signal exhibits an exaggerated, or peaked, transition.
One circuit for developing a signal proportional to the second derivative of a video signal includes two series connected, but isolated, RC circuits. A more sophisticated approach for developing a second derivative signal includes two series-connected delay lines for developing a first delayed video signal and a second, further delayed video signal. Next, the video signal is subtracted from the first delayed signal in a first differential amplifier, and the second delayed signal is subtracted from the delayed signal in a second differential amplifier. Finally, the signals from the two differential amplifiers are combined to produce a second derivative, or video enhancing, signal. This signal is also called a detail signal.
A major problem with video enhancers is that the differentiating process is quite susceptible to noise contamination. A noise pulse from the video signal would appear as a transition to the differentiation apparatus which produces a pair of complementary output pulses, or doublet, for each noise pulse present on the video signal.
A prior art device for removing, or "coring," the noise from the enhancement signal includes three series-connected biasing resistors connected between a first and second power supply potential. A first AC coupling capacitor couples the enhancement signal to the anode of a first diode having a cathode which is connected both to the juncture of the first and second biasing resistors and, by a second AC coupling capacitor, to an output terminal. The first AC coupling capacitor also couples the enhancement signal to the cathode of a second diode having an anode which is connected both to the juncture of the second and third biasing resistors and, by a third AC coupling capacitor, to the output terminal.
The biasing resistors develop potentials sufficient to reverse bias the diodes, preventing their conduction, except for enhancement signals having positive and negative excursions exceeding the biasing potential by the diode voltage drop. This circuit removes or "cores" the center noise-carrying portion of the enhancement signal. Although effective in reducing the noise contamination, coring techniques also eliminate low level enhancement signals, reducing the effectiveness of the enhancement process.